1. Field of the Invention
The present invention relates generally to a method of modifying a metal component of a self-supporting ceramic body, and to the modified product. More particularly, this invention relates to self-supporting ceramic bodies formed as the oxidation reaction product of a parent metal and having an interconnected metal-containing component modified in a post-forming step. The invention also relates to methods for producing such ceramic bodies.
2. Description of Commonly Owned Patent Applications and Prior Art
The subject matter of this application is related to U.S. Pat. No. 4,713,360 issued on Dec. 15, 1987 in the name of Marc S. Newkirk et al and entitled "Novel Ceramic Materials and Methods of Making Same." This patent issued from and commonly owned U.S. patent application Ser. No. 818,943, filed Jan. 15, 1986, which was a continuation-in-part of Ser. No. 776,964, filed Sept. 17, 1985, which was a continuation-in-part of Ser. No. 705,787, filed Feb. 26, 1985, which was a continuation-in-part of U.S. application Ser. No. 591,392, filed Mar. 16, 1984, all in the name of Marc S. Newkirk et al. and entitled "Novel Ceramic Materials and Methods for Making the Same." These applications disclose the method of producing self-supporting ceramic bodies grown as the oxidation reaction product from a parent metal precursor. Molten parent metal is reacted with a vapor-phase oxidant to form an oxidation reaction product, and the metal migrates through the oxidation product toward the oxidant thereby continuously developing a ceramic polycrystalline body which can be produced having an interconnected, metallic component. The process may be enhanced by the use of an alloyed dopant, such as in the case of an aluminum parent metal oxidized in air. This method was improved by the use of external dopants applied to the surface of the precursor metal as disclosed in commonly owned and copending U.S. Patent Application Ser. No. 822,999, filed Jan. 27, 1986, which is a continuation-in-part of Ser. No. 776,965, filed Sept. 17, 1985, which is a continuation-in-part of Ser. No. 747,788, filed June 25, 1985, which is a continuation-in-part of Ser. No. 632,636, filed July 20, 1984, all in the name of Marc S. Newkirk et al and entitled "Methods of Making Self-Supporting Ceramic Materials."
The subject matter of this application is also related to that of commonly owned and copending U.S. patent application Ser. No. 819,397, filed Jan. 17, 1986, and allowed on Aug. 1, 1988 which is a continuation-in-part of Ser. No. 697,876, filed Feb. 4, 1985, both in the name of Marc S. Newkirk et al. and entitled "Composite Ceramic Articles and Methods of Making Same". These applications disclose a novel method for producing self-supporting ceramic composites by growing an oxidation reaction product from a parent metal into a permeable mass of filler, thereby infiltrating the filler with a ceramic matrix.
Composite bodies comprising a metal boride, a metal component and, optionally, an inert filler are disclosed in U.S. Pat. No. 4,777,014, which issued on Oct. 11, 1988 in the names of Marc S. Newkirk et al and entitled "Process for Preparing Self-Supporting Bodies and Products Made Thereby." This patent issued from commonly owned U.S. patent application Ser. No. 837,448, filed Mar. 7, 1986, in the name for Marc S. Newkirk et al. and entitled "Process of Preparing Self-Supporting Bodies and Products Made Thereby." According to this invention, molten parent metal infiltrates a mass of a boron source, which may be admixed with an inert filler, and reacts with the boron source thereby forming a parent metal boride. The conditions are controlled to yield a composite body containing varying volume percents of ceramic and metal.
The entire disclosure of all of the foregoing Commonly Owned Patent Applications are expressly incorporated herein by reference.
Common to each of these Commonly Owned Patent Applications is the disclosure of embodiments of a ceramic body comprising an oxidation reaction product interconnected in one or more dimensions (usually in three dimensions) and one or more metallic constituents or components. The volume of metal, which typically includes non-oxidized constituents of the parent metal and/or metal reduced from an oxidant or filler, depends on such factors as the temperature at which the oxidation reaction product is formed, the length of time at which the oxidation reaction is allowed to proceed, the composition of the parent metal, the presence of dopant materials, the presence of reduced constituents of any oxidant or filler materials, etc. Although some of the metallic components may be isolated or enclosed, it is frequently the case that a substantial volume percent of metal will be interconnected and accessible from an external surface of the ceramic body. It has been observed for these ceramic bodies that this interconnected metal-containing component or constituent can range from about 1 to about 40 percent by volume, and sometimes higher as, for example, in the case of a boride composite.
In many applications for ceramic bodies having an interconnected, metal-containing component, the metal component contributes to, and may enhance, the properties of the ceramic body. In particular, the metal-containing component, due to its greater ductility, may contribute to the toughness or fracture resistance of the ceramic body. Likewise, the metallic constituent can be useful in providing a controlled degree of electrical conductivity in the ceramic body.
However, it also has been observed that for certain applications, the interconnected, metal-containing component may not provide the optimum properties for the intended use, and in some cases it can even detract from the performance of the ceramic body. For example, when the parent metal used in producing an alumina ceramic body is primarily aluminum and the resulting interconnected metal is substantially aluminum or aluminum alloy(s), it has been observed that although the ceramic body functioning at normal conditions may exhibit good fracture toughness or wear resistance, it can be degraded either by subjection or temperatures above the relatively low melting point of aluminum of approximately 660.degree. C., or by subjection to aqueous acid or alkaline environments which corrode away the aluminum constituent. It has been found that when the interconnected metal is so degraded, certain properties of the ceramic body, such as fracture toughness, strength, or wear resistance, are adversely affected. Likewise, in other product uses for such ceramic bodies, it has been observed that the interconnected metallic-containing constituent may not provide the optimum properties for the intended use, such as electrical conductivity, microhardness, etc.
It is known that graphite yarn can be impregnated with a metal matrix in order to improve certain properties of the yarn, and U.S. Pat. No. 3,770,488 to Pepper et al. discloses one such method for impregnating graphite yarn with an aluminum or magnesium matrix. In order to achieve proper wettability of the graphite yarn with the desired metal, the yarn is first infiltrated by another metal. The infiltrated yarn is then contacted with a molten bath of the desired metal impregnate which leaches out the metal infiltrate thereby forming a yarn reinforced by the desired metal matrix. This patent, however, is specific to graphite yarn, and further does not suggest that a polycrystalline ceramic body having residual interconnected metal can be modified in a post-forming operation.